Salt water and other feed fluids can be purified by applying the feed fluid under pressure to the outside of a bundle of hollow filter fibers that are packed into a cavity. The pure water, commonly referred to as filtrate, passes through the fiber walls and along the fiber passages, to a filtrate outlet for use as drinking water. The fibers typically lie in an elongated cavity, such as a cylindrical cavity that is seven inches in diameter and 80 inches long with the feed water pumped into one end of the cavity and concentrate removed at the opposite end. The walls of the cavity constitute one of many modules that are used in a system to supply the required flow capacity of filtrate, such as drinking water for a ship.
The pressure of the feed fluid drops along the length of the fiber bundle. As a result, a high pressure of water may have to be applied to the inlet end, or upstream end, of the cavity to assure there is sufficient pressure at the downstream end. A system that required water at lower pressure would be advantageous in many situations.
In some cases, a purifying system of given capacity must be as compact as possible, as in the case of many ships. This requires modules that can be closely stacked.